CPU 1511-1PN PID loops

Dear friends,

It has been a long time not bothering you
currently i am in  a midst of a special project that involves using the CPU 1511-1PN controller.

And i referred to the technical data of the above mentioned CPU.(and did not reached a solid conclusion) 

The project needs to run 48 PID loops, my question now could this CPU handle this number of PID loops?

Warm regards.
Tomy

Hello,

Sorry but there is no maximum number of PID loops.  The limit is based on memory of the PLC program size,cycle time...

Thanks you

Yes my friend, 
Sure i know that, but i am thinking loudly here, will the 48 PID loops cause a "glitches" or things will run smoothly?

Regards.

Tomy 

48 1 second PID loops, or 48 0.010sec (10ms) PID loops?

It matters.

How fast can the hardware respond, and how fast is the control output to be updated?

The best way to find out is to build it, leaving room in the s7-1500 rack to swap out the PLC with a higher powered one, test it and see if system performance is acceptable in every phase of the automation system.

Tomy Master

Yes my friend, 
Sure i know that, but i am thinking loudly here, will the 48 PID loops cause a "glitches" or things will run smoothly?

Regards.

Tomy 

Thanks for the kind reply,

In fact my costumer has the CPU and he can not change it now, and the PID loops are dedicated for temp control.
So in this case, do you recommend that execute (call it in a cyclic interrupt) the PID every 1 sec instead 10 ms periods?
Is it better for the CPU?

Warm regards.

Tomy 

Temperature loops are typically slow, so updating once a second is often quite possible.

However, some systems have very dynamic temperature swings and yet they want the overall average to be some given value.

And other systems have difficulty attaining setpoint and others have difficulty maintaining setpoint temperatures.

Each PID will need its own auto-tune with updates as appropriate for that PID loop.

On a system with 48 PIDs, you definitely want to start out with as long a cyclic interrupt span as possible and still maintain proper control.

Have the customer explain all 48 in groups of similar PID loops, then decide what update rate each group gets.

If all 48 are identical, make up 10 groups, and put 5 PID loops in all but the last group.  Each group represents 1 second update staggered by 100ms. (Cyclic interrupt #1 offset = 2ms, CI #2 offset = 102ms, CI#3->202ms...)

If they can all auto-tune (one at a time) and control once tuned, you are quite lucky given how little you know about the process.

Tomy Master

Thanks for the kind reply,

In fact my costumer has the CPU and he can not change it now, and the PID loops are dedicated for temp control.
So in this case, do you recommend that execute (call it in a cyclic interrupt) the PID every 1 sec instead 10 ms periods?
Is it better for the CPU?

Warm regards.

Tomy 

Great suggestions,

But how can we do the "offsetting" of Cyclic interrupt calling?

Best regards.

Tomy

Simplest way is to create 10 cyclic interrupt OBs and use the provided interrupt and offset values.

Most likely way it gets implemented (and in my case, I did it this way last year):

In the cyclic OB, create the 2ms offset (so it does not get called right at the "0" time when things tend to get triggered.

Call that cyclic interrupt OB, configure it to get called every 100ms, offset 2 ms.

In that OB, use a "IF PID_GROUP == n THEN" type of thing where n ranges from 1 to 10.

At PLC startup (OB100), set the PID_GROUP = 1.  At the end of cyclic OB, increment PID_GROUP, and check if it is > 10.  If so, set to 1.

Works very well.

Tomy Master

Great suggestions,

But how can we do the "offsetting" of Cyclic interrupt calling?

Best regards.

Tomy

Hi,

just because the original questions pops up every now and then in this forum, short answer to this:

The S7-1200/1500 PID control manual provides information on this. Generally the only limitations to amount of PID control functions on a PLC is memory and execution time. Values for both are provided in the manual, for each PID function, e.g. for PID Compact here (and for the others in the same document in the respective chapter): https://support.industry.siemens.com/cs/ww/en/view/108210036/158898449931

And with all the rest in this specific discussion...-> what 0_Einstein says ;)

Regards,
daviso

0_Einstein

Simplest way is to create 10 cyclic interrupt OBs and use the provided interrupt and offset values.

Most likely way it gets implemented (and in my case, I did it this way last year):

In the cyclic OB, create the 2ms offset (so it does not get called right at the "0" time when things tend to get triggered.

Call that cyclic interrupt OB, configure it to get called every 100ms, offset 2 ms.

In that OB, use a "IF PID_GROUP == n THEN" type of thing where n ranges from 1 to 10.

At PLC startup (OB100), set the PID_GROUP = 1.  At the end of cyclic OB, increment PID_GROUP, and check if it is > 10.  If so, set to 1.

Works very well.

Tomy Master

Great suggestions,

But how can we do the "offsetting" of Cyclic interrupt calling?

Best regards.

Tomy

 Dear Einstein, could you provide an example? The problem is that I have 64 PIDs and I observe that sometimes the PV value becomes equal to 0.5 degrees. This is repeated at different intervals. In some regulators more often, and in some less often. the duration of such a "failure" is 1-2 seconds. But I checked the values on the analog input - they are always within the normal range.